In recent years, large-sized high-resolution liquid crystal display panels have become popular not only for large-sized devices such as televisions but also for mobile terminals such as smart phones and tablet terminals. In a display device including a large-sized liquid crystal display panel, multiple driver ICs (integrated circuits) are often used to drive the liquid crystal display panel.
One factor to determine the display quality of such a liquid crystal display panel is the uniformity of grayscale voltages between or among the driver ICs which drive the source lines (which may be also referred to as data lines or signal lines) of the liquid crystal display panel. The grayscale voltages are a set of voltages used to convert digital image data into analog drive voltages.
Typical driver ICs are configured to supply voltages (which may be referred to as “grayscale reference voltages”, hereinafter) generated by voltage dividing by using a first voltage dividing resistor to a second voltage dividing resistor through buffer amplifiers (which may be referred to as grayscale amplifiers), and to generate a set of grayscale voltages by voltage dividing by using the voltage dividing resistor. The set of grayscale voltages are supplied to decoders (or D/A converters) for converting the image data into the drive voltages, and the decoders outputs the grayscale voltages selected in response to the graylevels of the respective pixels indicated by the image data. Output amplifiers are used to drive the source lines to the drive voltages corresponding to the grayscale voltages outputted from the decoders. In this configuration, if there are variations in the grayscale voltages generated in the respective driver ICs, block-shaped unevenness is undesirably generated in the display image, causing deterioration in the display quality.
One cause of the variations in the grayscale voltages between or among the driver ICs is a manufacturing variance of the grayscale amplifiers, especially, variations in the offset voltages of the grayscale amplifiers. Variations in the property of the grayscale amplifiers between or among the driver ICs undesirably generate variations in the grayscale voltages between or among the driver ICs.
One possible measure to address the variations in the grayscale voltages between or among the driver ICs, which are caused by the manufacture variance of the grayscale amplifiers, is to reduce the offset voltage of each grayscale amplifier. Various techniques have been proposed to reduce the offset voltage of an amplifying circuit. Proposed approaches include reduction of the manufacturing variance by optimizing the transistor size in the differential input stage of an amplifier, appropriate layout design and the like, and cancellation of the offsets in a pseudo manner by the circuit design; however, it is difficult to completely eliminate the variations in the property of the grayscale amplifier between or among the driver ICs.
Another possible measure to address the variations in the grayscale voltages between or among the driver ICs, which are caused by the manufacture variance of the grayscale amplifiers, is to connect interconnections used to transmit the grayscale voltages within the respective driver ICs (which may be referred to as “grayscale voltage lines”, hereinafter) by using interconnections provided on the liquid crystal display panel. This approach effectively reduces the variations in the grayscale voltages between or among the plurality of driver ICs; however, an unnecessary current may be generated between the driver ICs when there is a large difference in the grayscale voltage between or among the driver ICs, causing an increase in the current consumption. The increase in the current consumption due to generation of an unnecessary current is a significant problem for mobile terminals, such as cellular phones, smart phones and tablet terminals.
It should be noted that Japanese Patent Application Publications Nos. 2008-268473 A and 2008-258725 A disclose techniques for cancelling the offset of an output amplifier.
Also, Japanese Patent Application Publication No. 2008-111875 A discloses a technique for cancelling the offset voltage of an operational amplifier that is used as an output amplifier or grayscale amplifier in a pseudo manner.
Furthermore, Japanese Patent Application No. 2001-343948 A discloses a technique for offset cancelling in an output amplifier configured to generate a weight-averaged voltage of the grayscale voltages.
Furthermore, Japanese Patent Application No. 2001-188615 A discloses a technique for supplying an output voltage from an impedance conversion circuit (output amplifier) to a load capacitor without using an offset cancel circuit to generate a necessary charging voltage across the load capacitor.
Furthermore, Japanese Patent Application No. 2000-242233 A discloses a drive circuit of a display device, which selects a grayscale voltage in response to higher bits of digital image data and also controls the offset voltage of an output amplifier in response to the lower bits.